Apparatus and method for measuring power factor and torque on the output of variable frequency drives

ABSTRACT

An apparatus and method for sensing power factor and torque on the output of a variable frequency machine is disclosed. Initially, power is sensed in each phase of the three-phase machine by utilizing Hall generators for multiplying the phase current flowing through each phase by a current proportional to a phase voltage for each phase. The products are then summed to obtain a reading of the instantaneous power drawn by the three phase machine. The AC component is filtered out of the summed signal using a multistage filter which allows for maintaining a fast response time with low ripple. This output is divided by the speed of the machine and multiplied by a constant to result in a signal representing torque. For measuring power factor, power signal sensed in each phase is applied to a DC blocking capacitor, effectively separating the AC envelope proportional to voltage times current. The three signals are applied to absolute value amplifiers and summed to provide a signal yielding total power. The instantaneous power is then divided by this signal to provide power factor.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus and method for measuringpower factor and torque on the output of variable frequency drives.

Adjustable speed drives have been in use for several years and they arewidely used for controlling the speed of induction motors. Suchadjustable speed drives offer several advantages over fixed speed drivesFor example, adjustable speed drives increase the productivity ofindustrial machines since the machine speed can be selected for maximumoutput consistent with good product quality. Adjustable speed drivesalso make industrial machines more flexible so that when a productchange requires a different drive speed, the new speed is easilyselected. This benefit eliminates the need for gear or belt ratiochanges. Furthermore, electronic adjustable speed drive systems togetherwith process or programmable controllers enable the controlling ofmachine speed, fan speed or pump speed thereby further increasing theproductivity and versatility of any machine utilizing such adjustablefrequency drives.

In general, motor speed can be controlled by varying factors such asline frequency, the number of motor poles, and motor slip. Bycontrolling the motor speed by varying the frequency, a continuouslyvariable, highly efficient control throughout the entire speed range maybe achieved. Furthermore, such a control system is applicable to widelyused, three-phase squirrel-cage motors.

Variable frequency may be provided to input terminals of an AC motor inthe following manner. A main three-phase supply is first rectified andsmoothed in a rectifier or converter section. This DC power is then fedinto an inverter section, the current output of which approximates asine wave of variable frequency and amplitude.

Two different methods of obtaining the variable frequency output may beused. In a six step system, Pulse Amplitude Modulation (PAM), the DCvoltage obtained in the converter is varied. In a Pulse Width Modulationsystem (PWM), the frequency and the voltage are controlled by varyingthe pulse width within the invertor. The current output waveshape of thePWM system is superior to output wave forms produced by the PAM system.For both types of systems, the goal is to generate a current waveformthat approaches sinusoidal with the harmonic components of the waveformat a minimum to ensure minimized torque pulsation and temperature rise.In neither case, however, has this goal been completely met by knowndevices.

In many situations, the effectiveness of a drive can be furtherincreased by accurately measuring the power output. Measuring theinstantaneous power input to a machine or process provides a great dealof valuable information. This measurement which can be used as afeedback signal may be utilized to: automatically adjust the machinefeed rate; signal the beginning or end of a process; detect malfunctionsor problems; and, indicate, without contact, the flow rate, viscosity orpressure.

Load controls that sense power, have set points and analog outputs arewidely used in machine tools, chemical processes and material handling.Unfortunately, such load controls do not work on variable frequencysources. In order for a variable frequency power sensor to be ofpractical use as a machine controller, the sensor must have the abilityto: accurately measure power at both low and high frequencies; provide afast response with a low ripple and immunity to noise; and, have thecapability of working on both pulse amplitude modulation drives andpulse width modulation drives.

The main use of variable speed drives is to power induction motors. Tomeasure this power, the lag of the current behind the voltage (or powerfactor) must be considered. Traditional watt sensors rely on sensing thezero crossing of the sinusoidal voltage and current for power factorcalculation. Typical waveforms from PWM and PAM drives show that thewaveform is not clean enough for precise zero crossing measurement. Asecondary problem is the measurement of current. Many sensors use acurrent sensing toroid or a lamination transformer, but these devicesare not reliable at low frequencies. The combination of zero crossingand current measuring difficulties means that typical watt transducersdo not work on the output of a variable frequency drive.

The measurement of AC power requires the multiplication of voltage,current and a power factor so that the equation is:

    P=V×I×Cos φ

A simple and reliable method for performing this computationelectronically is by means of a Hall generator. A Hall generator is amagneto-sensitive semiconductor which, when driven by an electriccurrent and exposed to a magnetic field, generates a voltage that isproportional to the product of current and field. To utilize a Hallgenerator to measure power, a Hall device excitation current I isderived from a line voltage, and the phase load current produces aproportional field B in the magnetic circuit. The Hall generator exposedto this field generates an output voltage proportional to the product ofI, V, and the phase angle between them. The output contains an ACcomponent and a DC component. The AC component can be filtered out ifnecessary, and the expression for the DC component is V=k×I×B=k×V ×Icosφ=k×P where k is a constant representing the Hall voltage. The DC outputvoltage is therefore a measure of the AC power. With such additionalpower measurements as described above, only one or two phases aremeasured, and as a result, there is a large ripple component in theresulting output.

In three phase power measuring devices which are used for fixedfrequency power sensors, either one or two transducers are utilized tomeasure either one phase under the assumption that the load is balancedor two phases, respectively. A computer simulation of either of theseapproaches at various power factors shows that the output would have alarge ripple component which is unacceptable for control operations.

For many control applications, fast response is also critical. Typicalresponse time for watt transducers is 250 to 500 milliseconds. The slowresponse is due in great part to filtering circuits. For a power sensorto be useful the response time should be reduced to about 15milliseconds.

A power transducer must also live in close proximity to the variablefrequency drive, and such drives generate a great deal of RF noise fromthe high frequency switching. Therefore, both the housing of the sensorand the internal circuitry should be designed to minimize RF noise.

In addition to sensing power of the variable frequency drive, it isoften important to measure the torque produced by the drive. Torque isequivalent to the horsepower divided by the speed multiplied by aconstant. After measuring the power produced by the variable frequencydrive, a measurement of the speed of the drive through a speedtransducer and dividing this into the horsepower allows one to determinethe torque. Existing schemes for measuring torque are usually mechanicalor implemented electronically with slow power transducers. Torquemeasurements are important in industry as they form the basis forunderstanding many mechanical phenomenon, and in particular are used tocharacterize electric motor performance.

Relative to electric motors, mechanical torque measurement methods fallinto three general categories:

1. All mechanical.

2. Electronic strain gauges mounted on mechanical members.

3. Eddy current brakes.

In case 1, the classic Pony brake applies a friction load to the outputshaft by means of woodblocks, flexible bands, or other friction surfacedevices. The torque is then measured by balancing the outputs againstweights applied to a fixed lever arm. In case 2, a strain gauge ismounted directly to the shaft transmitting the power in the load. Theshaft twists as a function of torque, the strain gauge deforms and avoltage output proportional to the torque results. In case 3 a rotatingmetal disc in a magnetic field induces eddy currents in the disc. Thesecurrents dissipate as heaL providing a value equal to horsepower; atachometer provides a speed reference and division of the two results intorque measurement.

Methods 1 and 3 discussed above work fine for measuring torque but arenot practical in all applications. The devices required to implementthese methods are physically large, measure the torque output veryslowly, and are cumbersome to implement. Strain gauges can be made quitesmall and do respond rapidly, however, they have reliability problemsassociated with the wiring to the resistive bridge since the bridge ismounted on a rotating surface. Schemes for brush pickups or RF orinductive coupling have been used, but result in increased cost, slowerresponse, more complexity and greater physical size. Installation costsare also high since it usually requires modification of the machine.

Another useful measure of variable frequency drive performance is powerfactor. There are many methods to compute power factor, but they allhave shortcomings Generally they only work at one frequency (e.g. 60Hz),look at only one phase of a three phase system and only work withsinusoidal waveforms. With variable speed drives, distorted wave shapesare common and do not fare well with conventional power factormeasurement techniques. Furthermore, most systems use a great deal ofdamping so that rapid or instantaneous measurements of power factor arenot possible.

It is therefore a principal object of the present invention to providean apparatus and method for sensing torque and power factor of avariable frequency drive that is accurate, reliable and which providesan output signal that does not exhibit a large ripple component.

It is another object of the present invention to provide an apparatusand method for measuring torque and power factor of a variable frequencydrive which will be sensitive at both low and high frequencies and willprovide a fast response time.

A further object of the present invention is to provide an apparatus andmethod for measuring torque and power factor of a variable frequencydrive which Will provide independent and precise machine control andprotection.

A still further object of the present invention is to provide anapparatus and method for sensing torque and power factor of a variablefrequency drive which will provide a linear output and which isextremely forgiving to gross overloads.

Yet another object of the present invention is to provide a system fordetermining torque on either fixed frequency sources or variable speeddrives.

A still further object of the present invention is to provide a methodto compute power factor which works at more than one frequency.

Still another object of the present invention is to provide a method tocompute power factor that will work on a variety of waveforms.

Yet another object of the present invention is to provide a method tomeasure power factor that is essentially instantaneous.

SUMMARY OF THE INVENTION

In accordance with the objects of the invention, an apparatus and methodfor measuring power factor and torque output on a variable frequencydrive includes a means for measuring power in each phase of a threephase machine. The power is measured by taking the phase voltage fromeach phase, and converting the voltage into a control current. Thiscontrolled current and the appropriate phase is then utilized to drive aHall generator so that the Hall generator multiplies the control currentsignal and the phase current signal to obtain an output signal thatrepresents power (V×I cos φ ) in that particular phase. The powermeasurements from the three phases are summed to provide a measurementof the instantaneous power drawn by the three phase machine This powersignal includes an AC component which is filtered out by a multistagefilter and which allows the apparatus to maintain a rather fast responsetime. Preferably the multistage filter utilizes RC filters to performthe filtering operation.

Power Factor

After determining the power in each phase of the three phase motor, thethree VI signals for each phase are summed; each is also applied toabsolute value amplifiers. Dividing the power by the DC isolatedabsolute value of the same power instantly yields the power factor atthat particular moment for the machine.

Torque

Speed measurements are made by attaching tachometers to the machine.Power and speed are divided which results in a signal proportional totorque. In addition, a circuit is provided to insure that division byzero cannot occur by setting a small threshold, typically 1/100 of thescale below which the denominator is clamped. In the case there is zerospeed at zero power the numerator is zero and the output is zero. In thecase of zero speed at full power (locked rotor), the result should be aninfinite output as the denominator is taken to the zero limit, but as apractical matter, a full scale output accomplishes the same thing. Thedenominator of 1/100 full scale insures that a full scale reading wouldresult in all real world conditions.

These and other features and objects of the present invention will bemore fully understood from the following detailed description whichshould be read in light of the accompanying drawings in whichcorresponding reference numerals refer to corresponding parts throughoutthe several views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the circuitry for obtaining the powermeasurement in each of the three phases of the present invention;

FIG. 2 is a schematic diagram of the multistage filter of the powersensor of the present invention.

FIG. 3 shows a block diagram for measuring torque.

FIG. 4 is a schematic diagram of the multistage filter of the torquesensor of the present invention.

FIG. 5 is a block diagram for measuring power factor.

FIG. 6 is a schematic of the power factor sensor of the presentinvention.

FIGS. 7A-7E show voltage and current graphs for measuring power factor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, the variable frequency power sensor of thepresent invention obtains a measure of a variable frequency power byindependently measuring the power in each of the three phases and thensumming the three power values. By measuring all three phases, theripple is substantially reduced. The AC component of the resulting powermeasurement is filtered from the output by a multistage filter which isprincipally shown in FIG. 2.

Turning first to the phase measuring circuitry principally shown in FIG.1, while the excitation for a Hall generator may be derived directlyfrom phase voltage for a particular phase of a multi-phase machine by avoltage to current converting resistor, several problems would resultwith such a circuit. Principally, high power dissipation would occur,and there would be a high and potentially lethal leakage current sincethe current to ground from each phase is the excitation of the cell(typically 40 mA). To avoid these two problems, a high value resistor 14is provided to limit the leakage current to microamps, and as a result,the circuit will exhibit a lower power dissipation (approximately 0.25watts). Resistor 14 will also facilitate the normalizing of the rangevoltage with the divider resistor 16. Capacitor 18 is provided to supplyhigh frequency noise filtering and op-amp 20 which is characterized by ahigh breakdown voltage and good common mode characteristics will alsohave a slew rate which will provide some high frequency noise filtering.Only a minimal amount of filtering is required to preserve the accuracyof the reproduced waveform presented to the Hall cell. A LM324operational amplifier as manufactured by National Semiconductor issuitable for this purpose.

The op-amp 20 output is sent through resistors 22, 24 to the base oftransistors 26, 28 which are provided to buffer the op-amp outputtransistors. The output from the two transistors are combined at summingpoint 30, and this output signal provides a negative feedback voltagethrough variable resistor 32 to the inverting input of the op-amp 20.The feedback resistor 32 is a variable resistor because this resistor isused for Hall cell calibration. The output voltage from the summingpoint 30 passes through resistor 34 which may be a temperaturecompensating resistor for Hall cell stabilization. Resistor (andconsequently gain) values are computed to prevent damage to the Hallcell from gross overloads.

The Hall generator 36, which is supplied with the total current of aparticular phase, is modulated by the appropriate excitation current toproduce a power proportional signal. A suitable Hall device willsaturate at a fixed current, and an example of such a Hall generator isthat manufactured by FW Bell under the designation PI100 and PI350.

Variable resistor 38 balances the bridge voltage of the Hall cell 36 andthe variable resistor 38 may be calibrated dynamically by oscilloscopeobservation of Sin² ωt. Output voltages of the Hall cell 36 are suppliedthrough resistors 40 and 42 to the inverting and non-inverting inputs ofop-amp 43 respectively. Op-amp 43 is a low noise, high speed op-ampwhich is configured for differential gain to take advantage of commonmode rejection. A suitable op-amp would be a LM725 manufactured byNational Semiconductor. Resistors 44 and 46 are selected to set anappropriate gain, and the combination of resistor 48 and capacitor 50set compensation and provide a certain amount of filtering. Resistor 52nulls the amplifier 43. The output of the amplifier 43 provides a signalrepresenting the power in the particular phase to which it is connected.

The phase signals from each of the three phases must be summed, andamplifier 60 is provided for this purpose. As discussed above, the ACcomponent must be filtered from the output wave, and once the signalsfrom three phases are summed by amplifier 60, the voltage signalrepresenting the sum is filtered to remove the AC component. In order tomaintain a fast response time, a multistage filter is utilized whichwill provide a response time of approximately 15 milliseconds. The firststage of filtering is effected by an RC filter circuit comprisingcapacitor 62 and resistor 64. Op-amp 60 will provide a voltage sourcesignal for the next RC stage comprising capacitor 66 and resistor 68. Byutilizing the amplifier 60 to provide the voltage source for the RCfilter comprising capacitor 66 and resistor 68, the overall constructionis simplified and the attenuation effects normally associated withmultistage RC filters are thereby reduced. In the preferred embodimentshown in FIG. 2, there are four stages of filtering which will enablethe rapid decrease of ripple per stage with a very slow increase in theresponse time. In the remaining filtering stages, amplifier 70 providesa voltage source for the RC circuit comprising capacitor 72 and resistor74, and op-amp 76 will feed a voltage signal to the RC circuitcomprising capacitor 78 and resistor 80. Amplifier 82 is used to bufferthe last filter stage, and the RC circuit comprising capacitor 84 andresistor 86 is provided to increase the stability phase margin whendriving long capacitive cables.

In a preferred embodiment there are two output signals modes. One willprovide an output from 0 to approximately -7 volts which is used foradditional signal processing by other "black boxes". If the device isused for a regenerative motor, the polarity reverses to 0 to +7 voltsfull scale. In a stand-alone mode which is used for power indicationonly, amplifier 88 is provided for transducer output scaling andadditional filtering. Resistors 90 and 94 set the gain and therefore thescaling along with inverting amplifier 88. Resistor 90 and theequivalent of DPDT switch 100 is located on a plug-in module. Switch 100selects whether the output signal comes from amplifier 82 or 88.Resistor 94 and capacitor 92 provide one more stage of filtering in thetransducer stand alone mode. Resistor 98 and capacitor 96 decoupleamplifier 88 from capacitive loads. Scaling may be provided by a plug-inmodule as well as by model selection. Typically the output is scaled to0 to +10 volts or 0 to +5 volts full scale. Polarity reverses (0 to -10volts, 0 to -5 volts full scale) for regenerative motors.

With respect to the power supply for the present invention, anyconventional power supply may be utilized. In one embodiment, the powersupply may be a split supply which will provide for op-amp ripplerejection and optimization of common mode rejection in amplifiers 43.Rail voltages can be chosen to provide low dissipation for the op-ampsand to limit the output signal in the event of gross overloads throughthe Hall cell. This allows the Hall cell 36 to be greatly undersized toprovide maximum sensitivity at a small part of the machining cyclewithout the fear of damage at higher normal cycle loads.

Torque Measurement

Torque is equal to horsepower divided by the speed multiplied by aconstant. The schematic shown in FIG. 1 may be coupled with a speedtransducer and a divider circuit to calculate torque. Speed measurementsmade by attaching tachometers to the machine or motor is a relativelynoninvasive machine procedure. Power measurement by the circuit shown inFIG. 1 and described above and processing of the resulting signalcompletes the system and results in the signal proportional to torque.Power transducers are not new but as pointed out in FIG. 1, theschematic works with either fixed frequency sources or variable speeddrives.

As shown in FIG. 3, the speed signal may come from one or two sources,i.e., either tach generator 101 or from the variable speed drive 102.Tach generators 101 are common, and with fixed frequency applicationsthey represent the best approach. Output from a tachometer is generallyin the 7 volts per 1000 rpm range, and input conditioning is provided inthe first stage. Input conditioning is noise filtering and voltage levelscaling of the input signal to match the following circuits. Whenvariable speed drives are present, another route is possible. Drives aregenerally equipped to provide a voltage output signal (typically 1 to 5volts) that is proportional to the frequency of the motor drive. In thecase of a synchronous motor, this is exactly equal to the rotor speed;in the case of an induction motor, there is a small error due to slip.The slip error may generally be disregarded. The benefits of using thesignal are increased system reliability (no mechanical tachometer) andinstallation simplicity. Signal conditioning and scaling are provided bythe appropriate input stage, that is, it is switch selectable by switch103.

The problem of zero speed of the variable frequency drive or motorcreates a problem as division by zero is not possible. When zero speedoccurs, two possibilities exists:

(1) zero speed at zero power; and

(2) zero speed at full power.

A circuit is provided as shown as denominator damp 105 in FIG. 3 toinsure that division by zero cannot occur by setting a small threshold,typically 1/100th of scale, below which the denominator is clamped. Incase number 1, the numerator is zero and the output is zero. In casenumber 2, the result would be an infinite output as the denominator istaken to the zero limit; however, as a practical matter, a full scaleoutput accomplishes the same thing. A denominator of 1/100th of scaleinsures that full scale would result in all real world conditions.

Power measurement is made as described in FIGS. 1 and 2. The electricalcircuit is represented schematically by the area enclosed in the dottedline and labeled PH-3 in FIG. 3. A small change is made in the filteringstage of FIG. 2 by omitting the integrating capacitor 62. Changing thefilter stage results in an extremely fast response time. To maintain thefour stage filter in the filtering stage, another filter stage is addedto the filter block 99 and 15 shown in FIG. 4. The selector switch 104in FIG. 3 couples either the instantaneous value or the filtered valueto the divider 98. This block divides the power measurement by the speedand multiplies this total by a constant to give the instantaneous oraverage torque in desired units (Ft-lbs, Newton-meters, etc.). As inFIG. 2, the filter in FIG. 4 provides very low ripple with fast responsetime, approximately 15 milliseconds. Full scale voltage and current areadjusted by plug-in networks comprised of components 16, 44, 46, 48, and50, shown in FIG. 1. Changing resistors 16 changes the voltage (either230 volt, 460 volt or greater), changing the others affects the current.These plug-ins make field changes simple and accurate as discussedabove.

Scaling for units is provided at the divider 98. The output is a signalproportional to torque.

Power Factor

Power factor is the phase difference between the line voltage and thecurrent. It is expressed as a number between zero and one; oneindicating a totally resistive load, and zero indicating a totallyreactive load. The power factor is mathematically expressed as cos φ.Totally reactive loads have a phase shift of 90 degrees (cos φ=0), whileresistive loads have a shift of 0 degrees (cos φ=1). Totally reactivecapacitive loads cause the current to lead the voltage by 90 degreeswhile totally reactive inductive loads cause the current to lag thevoltage by 90 degrees. Power in watts for a three phase line isexpressed as the product of voltage, current and power factor and thesquare root of 3. That is, power equals voltage multiplied by currentmultiplied by power factor multiplied by the square root of three. Sincepower factor is the ratio of real power to total power, the voltage,current and square root of three must be eliminated from the equation.The ratio is expressed from zero to one, is dimensionless and ismathematically represented as PF=(V· I· Cos·φ·√3 ) divided by (V·I·√3 ).

Since the Hall device measures the strength of a magnetic field presentinside of a torridal flux concentrator, the magnetic flux in the torridgap is proportional to current. To generate an output signal, the Halldevice must be excited by an external current. The Hall voltage outputis the product of these two signals. The output signal is generallysmall (millivolts), essentially independent of frequency (DC to MHz insome designs) and by definition considers the phase of the two inputsignals.

FIG. 5 is a block diagram showing the measurement of power factor. Thenumbers in FIGS. 5 and 6 are the same for the same components found inFIGS. 1 and 2.

As discussed, the Hall cell 36, the excitation circuit 20 and theamplification 43 are shown in FIG. 1. This is represented by block 110in FIG. 5 and schematically in FIG. 6. Filter stages 130, shown in FIG.5, are the same as that disclosed in FIG. 2 with the exception thatcapacitor 62 is omitted. Since it is necessary that four stages offiltering be included, and since capacitor 62 is one of those stages,the filter string needs to be increased by one. This is accomplished byinserting another set of components 66, 68 and 70 as shown in FIG. 4.

The basis for this invention lies in the way the signals representingvoltage and current are handled after they are multiplied. For thisdiscussion, it is assumed that both the currents and the voltages arerepresented by sinusoidal waveforms.

Voltage (FIG. 7A): V=V·sin(2πf)

Current (FIG. 7B): I=I·sin(2πf+α), where α=phase angle.

Line voltage is presented to the Hall cell as excitation and the currentis measured as the flux density in the core. Due to the physics of theHall cell, it acts as a multiplier; the resultant Hall output voltage isproportional to power:

    P=V·sin(2πf)·I·sin(2πf+α) (1)

The √3 term is now introduced as a constant, and the terms rearranged:

    P=√3·V·I [sin(2πf) sin(2πf+α)](2)

For a resistive load α=0 degrees, PF=1, and power is:

    P=√3·V·I·sin.sup.2 (2πf) (FIG. 7C) (3)

For a fully reactive lagging load, α=-90 degrees and PF=0. Since sin(2πf-π/2)=-cos(2πf), then:

    P=√3·V·I·sin(2 πf)·[-cos(2 πf)](FIG. 7E)                                          (4)

Note from FIG. 7D that the V-I envelope has developed a DC offset in thenegative direction relative to FIG. 7C. This offset is due to the cos(2πf) term in Equation 4. Most importantly, note that the amplitude of theV-I envelope remains constant regardless of phase shift. This amplitudecan be shown to be:

    Amplitude=√3·V·I·[1/2 sin (4 πf) ](5)

Equation 5 shows that eliminating the DC component from the output ofthe Hall cell results in a quantity proportional to V·I independent ofpower factor. The DC component can be blocked by a capacitor. Withappropriately scaling, dividing the signals before and after thecapacitor yields power factor.

Before capacitor:

    V.sub.signal =√3·V·I cos π     (6)

After capacitor

    V.sub.signal =√3·V·I              (7)

Dividing and cancelling terms: ##EQU1##

By definition

    cos π=PF                                                (9)

To understand implementation of the device, refer to the block diagramon FIG. 5. The output of op-amp 43 is the output of the cell, equal toVI cos φ. This signal is applied to a DC blocking capacitor (C1),effectively separating the AC envelope proportional to VI. Since this isintended for a three phase line, all three phases are monitored; thishas the advantage of showing the total instantaneous power factor.Simply summing the three V·I signals would cause the output to be zeroas the three lines have a 120 degree phase shift. To avoid this, thethree outputs are applied to absolute value amplifiers 120. Summing bothVI cos φ and VI has a distinct advantage with three phase lines in thatthese values sum to a constant DC level (any ripple left in the signalis due to line phase imbalances). Applying the signals to a divider 125yields power factor computations instantaneously (Eqs 7-9). The feed tothe divider from the V I summer 121 may also be taken after the fourstage filters shown in FIG. 5 for averaging out line noises and stillmaintaining fast response times (15 milliseconds).

Note that the above discussion applies equally as well to otherwaveshapes and frequencies. Once again, the only changes to circuits ofFIGS. 1 and 2 are the omission of the first VI cos φ integratingcapacitor 62 on op-amp 60. To maintain the 4 stage filtering shown inFIG. 2, one more stage has been added to the remaining filter stages inFIG. 4. In addition, full scale power voltage and current ranges arechanged by plug in networks. Voltage networks contain resistor #16; thissets the full scale line voltage (230v, 460v, etc). Currents are set bycomponents 44, 46, 48 and 50; for example 10A, 30A, 100A. Thisarrangement makes field changes simple and accurate. Networks (ormodules) are discussed above.

While the foregoing invention has been described with reference to itspreferred embodiments, various alterations and modifications may occurto those skilled in the art. All such alterations and modifications areintended to fall within the scope of the appended claims.

What is claimed is:
 1. A torque sensor for determining torque through awide bandwidth of frequencies, continuously and automatically, at anymoment in time in a three-phase AC machine with speed to produce aresulting signal, said resulting signal representing sensed torque as ananalog function, said torque sensor producing said signal withinmilliseconds, said sensor comprising:means for measuring power in eachphase of said three-phase AC machine, said means for measuring powerbeing capable of measuring distorted and non-distorted sinusoidal andnon-sinusoidal wave shapes while maintaining sensitivity at high and lowfrequencies, said means for measuring power including:means fordetecting a phase current from said phase of said three-phase AC machineto obtain a phase current signal; means for detecting a phase voltagefrom said phase of said three-phase AC machine to obtain a phase voltagesignal; means for amplifying said phase voltage signal, said means foramplifying said phase voltage signal also acting to convert said phasevoltage signal into a control current proportional to the phase voltagesignal in amplitude and phase, said control current being temperaturecompensatable by means of a temperature compensating resistor; and aHall generator driven by said control current, said Hall generatorconfigured and adapted to sense a magnetic field generated by the phasecurrent signal, said Hall generator providing an output signalindicative of instantaneous power drawn by said phase of saidthree-phase AC machine; said three-phase AC machine providing a summedoutput signal, said summed output signal being composed of a DCcomponent and an AC component; means for filtering said AC componentfrom said summed output signal of said three-phase AC machine providinga filtered linear DC output signal, said means for filtering includingat least four consecutive stages of filters, each of said stages offilters including an RC filter, said means for filtering capable ofmaintaining a ripple of about 1%; means for measuring speed of the ACmachine; means for dividing the filtered linear DC output signal by themeasured speed to provide an output proportional to torque of themachine.
 2. The torque sensor according to claim 1 with means forcompensating the measured speed such that the compensated measured speedis not permitted to equal zero.
 3. The torque sensor according to claim1 wherein the means for measuring speed of the AC machine produces afrequency proportional signal from the variable speed drive.
 4. Apower-factor sensor for sensing power-factor through a wide bandwidth offrequencies, continuously and automatically, at any moment in time in athree-phase AC machine to produce a resulting signal, said resultingsignal representing power-factor, said power-factor sensor producingsaid signal within milliseconds, said sensor comprising:means formeasuring power in each phase of said three-phase AC machine, said meansfor measuring power being capable of measuring distorted andnon-distorted sinusoidal and non-sinusoidal wave shapes whilemaintaining sensitivity at high and lower frequencies, said means formeasuring power including:means for detecting a phase current from saidphase of said three-phase AC machine to obtain a phase current signal;means for detecting a phase voltage from said phase of said three-phaseAC machine to obtain a phase voltage signal; means for amplifying saidphase voltage signal, said means for amplifying said phase voltagesignal also acting to convert said phase voltage signal into a controlcurrent proportional to the phase voltage signal into a control currentproportional to that phase voltage signal in amplitude and phase, saidcontrol current being temperature compensatable by means of atemperature compensating resistor; and a Hall generator driven by saidcontrol current, said Hall generator configured and adapted to sense amagnetic field generated by the phase current signal, said Hallgenerator providing an output signal indicative of instantaneous powerdrawn by said phase of said three-phase AC machine; said three-phase ACmachine providing a summed output signal, said summed output signalbeing composed of a DC component and an AC component; means forfiltering said AC component from said summed output signal of saidthree-phase AC machine providing a filtered linear DC output signal,said means for filtering including at least four consecutive stages offilters, each of said stages of filters including an RC filter, saidmeans for filtering capable of maintaining a ripple of about 1%; meansfor blocking the DC component of each phase voltage signal and applyingeach output to an absolute value amplifier to produce a signalproportional to voltage multiplied by current; means for summing eachsignal proportional to voltage multiplied by current to produce a totalpower signal; means for filtering said total power signal, said meansfor filtering including at least four consecutive stages of filters eachof said stages including an RC filter, said means for filtering capableof maintaining a ripple of approximately 1%; and means for dividing thesignal by the filtered linear DC output signal to produce a signalrepresenting average power-factor.
 5. A power factor sensor for sensingpower-factor through a wide bandwidth of frequencies, continuously andautomatically, at any moment in time in a three-phase AC machine toproduce a resulting signal, said resulting signal representing sensedpower-factor as an analog function, said power-factor sensor producingsaid signal within milliseconds, said sensor comprising:means formeasuring power in each phase of said three-phase AC machine, said meansfor measuring power being capable of measuring distorted andnon-distorted sinusoidal and non-sinusoidal wave shapes whilemaintaining sensitivity at high and low frequencies, said means formeasuring power including:means for detecting a phase current from saidphase of said three-phase AC machine to obtain a phase current signal;means for detecting a phase voltage from said phase of said three-phaseAC machine to obtain a phase voltage signal; means for amplifying saidphase voltage signal, said means for amplifying said phase voltagesignal also acting to convert said phase voltage signal into a controlcurrent proportional to the phase voltage signal in amplitude and phase,said control current being temperature compensatable by means of atemperature compensating resistor; and a Hall generator driven by saidcontrol current, said Hall generator configured and adapted to sense amagnetic field generated by the phase current signal; said Hallgenerator providing an output signal indicative of instantaneous powerdrawn by said phase of said three-phase AC machine; means for blockingthe DC component of each phase voltage signal and applying each outputto an absolute value amplifier to produce a signal proportional tovoltage multiplied by current; means for summing each signalproportional to voltage multiplied by current to produce a total powersignal proportional to VI; said three-phase AC machine providing asummed output signal, said summed output signal being composed of a DCcomponent and an AC component; and means for dividing the absolute powersignal by the summed output signal to provide a signal representinginstantaneous power factor.
 6. A sensor for power-dependent parameters,said sensor comprising:means for measuring power in each phase of athree-phase AC machine having speed, said means for measuring powerbeing capable of measuring distorted and non-distorted sinusoidal andnon-sinusoidal wave shapes while maintaining sensitivity at high andlower frequencies, said means for measuring power including:means fordetecting a phase current from a phase of said three-phase AC machine toobtain a phase current signal; means for detecting a phase voltage froma phase of said three-phase AC machine to obtain a phase voltage signal;means for amplifying said phase voltage signal, said means foramplifying the phase voltage signal also acting to convert the phasevoltage signal into a control current proportional to the phase voltagesignal in amplitude and phase, said control current being temperaturecompensatable by means of a temperature compensating resistor; and aHall generator driven by said control current, said Hall generatorconfigured and adapted to sense a magnetic field generated by the phasecurrent signal, said Hall generator providing an output signalindicative of instantaneous power drawn by said phase of saidthree-phase AC machine; said three-phase AC machine providing a summedoutput signal, said summed output signal being composed of a DCcomponent and an AC component; and means for filtering said AC componentfrom said summed output signal of said three-phase AC machine providinga filtered linear DC output signal, said means for filtering includingat least four consecutive stages of filters, each of said stages offilters including an RC filter, said means for filtering capable ofmaintaining a ripple of about 1%.
 7. The sensor of claim 6 where thepower-dependent parameters are torque and power-factor.
 8. A sensor inaccordance with claim 6, wherein the power-dependent parameter istorque, said sensor sensing torque through a wide bandwidth offrequencies, continuously and automatically, at any moment in time inthe three-phase AC machine to produce a resulting signal, said resultingsignal representing sensed torque as an analog function, said sensorproducing the resulting signal within milliseconds, said sensorcomprising:means for measuring speed of the AC machine; means fordividing the filtered linear DC output signal by the measured speed toproduce an output signal being proportional to said sensed torque of theAC machine.
 9. The torque sensor according to claim 8 with means forcompensating the measured speed such that the compensated measured speedis not permitted to equal to zero.
 10. The torque sensor according toclaim 8 wherein the means for measuring speed of the AC machine producesa frequency proportional signal from the AC machine.
 11. The torquesensor according to claim 10 wherein the AC machine is a variablefrequency drive.
 12. A sensor in accordance with claim 6, wherein thepower-dependent parameter is power-factor, said sensor sensingpower-factor through a wide bandwidth of frequencies, continuously andautomatically, at any moment in time in a three-phase AC machine toproduce a resulting signal, said resulting signal representing sensedpower-factor, said power-factor sensor producing said signal withinmilliseconds, said sensor comprising:means for blocking the DC componentof each phase voltage signal and applying each output to an absolutevalue amplifier to produce a signal proportional to voltage multipliedby current; means for summing each signal proportional to voltagemultiplied by current to produce a total power signal; means forfiltering said total power signal, said means for filtering including atleast four consecutive stages of filters, each of said stages includingan RC filter, said means for filtering capable of maintaining a rippleof approximately 1%; and means for dividing the signal by the filteredlinear DC output signal to produce an output signal being representativeof sensed average power-factor of the AC machine.
 13. A sensor inaccordance with claim 6, wherein the power-dependent parameter ispower-factor, said sensor sensing power-factor through a wide bandwidthof frequencies, continuously and automatically, at any moment in time ina three-phase AC machine to produce a resulting signal, said resultingsignal representing sensed power-factor being a continuous analogfunction, said power factor sensor producing said signal withinmilliseconds, said sensor comprising:means for blocking the DC componentof each phase voltage signal and applying each output to an absolutevalue amplifier to produce a signal proportional to voltage multipliedby current; means for summing each signal proportional to voltagemultiplied by current to produce a total power signal; means fordividing the absolute power signal by the summed output signal toproduce an output signal being representative of sensed instantaneouspower-factor of the AC machine.